1. Field of the Invention
The subject of the invention is a process for recording semihalftone images. In this process, an original is analyzed by scan-resolution and divided into a number of scan fields each with k scan dots, the average tonal value of each scan field is determined, and the original is reproduced on a recording medium with image resolution effected by individual recording dots with the tone value "print" (on) or "no-print" (off).
2. Description of the Related Art
With this type of process, originals, for example, black and white transparencies, can be reproduced by dots recorded on a recording medium, such as, for example, a sheet of paper, a printing plate of a fluorescent screen. The recorded dots can be only black or white. The various shades of gray ranging from black to white in an original are achieved on the recording medium by varying the black recording dots as a function of the desired gray steps. This means that all recording dots of a black area of the original are black in the reproduction, while the proportion of white recording dots located among the black recording dots increases with decreasing blackness. For the light grey areas of the original, the recording medium shows a low frequency of black recording dots, until ultimately, all recording dots are white, that is, not printed, for the white areas.
There are known process in which the individual recording dots of the gray scale of the original are correspondingly more or less toned. With such processes, each step of the grey scale can be reproduced on the recording medium. However, if the original should be reproduced by means of recording dots that can take the tone value of either "print" (black) or "no-print" "empty" (white), the use of the above image recording method is not possible. The various gray shades can also be reproduced with recording dots that can take only two tone values in two ways. The dots may be placed on constant center distances but vary in size thus covering more or less of a given area and creating the appearance of the various densities of a gray scale. This is the traditional halftone imaging system widely accepted by the printing industry. Another way of creating the various gray levels is by varying the area frequency of print (or no-print) recording dots of a constant size. Hence, these are known as "semihalftone images" in which the gray steps of the gray scale are reproduced according to a previously selected classification into gray gradations. The processing of such semihalftone images with the aid of the digital technique is very simple, because the binary method can be used for image processing ("1" for a print and "0" for a no-print image dot).
A process for the recording of semihalftone images is known from EP-A-0 132 453. This process minimizes errors in the digital processing of electronic signals and is illustrated with the aid of an image recording process. In the known process, an original is analyzed by scanning linewise and dotwise within each line. The gray shade or generally, the tonal value for each scan dot of the original is determined and stored. The tonal value of a scan dot is determined by means of a tonal value scale subdivided into 256 tonal value steps. A scan field consists of a number of scan dots arranged in a matrix and is reproduced on a recording medium by a specific number of recording dots which are arranged in the form of a matrix and which can take either the tone value "print" or the tone value "no-print".
In the known process, a scan dot of the original is supposed to be reproduced by 16 recording dots on the recording medium. This means that the 256 tonal values of the original are reproduced by 16 tonal values on the recording medium. The 16 gray steps in the reproduction results from a number of the 16 recording dots being printed, this number corresponding to the tonal value steps of the scan dots. In the conversion of the tonal value steps of the original into those tonal value steps, which are reproduced on the recording medium, relatively large errors occur. In order to minimize the errors in the reproduction, more scan dots are sometimes collected into a scan field, the average tonal value step of which is reproduced as accurately as possible (average tonal value step being the average of the tone values of all scan dots of the scan field). Each individual scan dot of the scan field can thus be reproduced with quite a relatively large error, while the average tonal value step of the scan field is reproduced relatively accurately.
The known process shows very extensive calculations to minimize errors, which are made in the conversion of the tonal value steps of the originals into those of the reproduction medium. In addition, large matrices with data must be processed. Furthermore, the resolution capability of images recorded by this method is limited by the size of the scan dots, with correspondingly more recording dots in the reproduction.